1. Field of the Invention
The present invention relates to a novel copolymer produced by using a microorganism, and a process for producing the same. More specifically, it relates to a novel terpolymer comprising 3-hydroxybutyrate units (hereinafter sometimes abbreviated as 3HB), 3-hydroxyvalerate units (hereinafter sometimes abbreviated as 3HV), and 4-hydroxyvalerate units (hereinafter sometimes abbreviated as 4HV), and a process for producing the same.
2. Description of the Related Art
Since poly-3-hydroxybutyrate (PHB) is accumulated within cells of a large number of microorganisms, as an energy storing substance, and is a thermoplastic polymer exhibiting an excellent biodegradability and biocompatibility, it has attracted attention as a "clean" plastic which protects the environment. Particularly, under the present situation in which synthetic plastics are a serious social problem, from the standpoints of environmental pollution and resource circulation, PHB also has attracted attention as a biopolymer which does not depend on petroleum.
For example, PHB can be applied for medical materials such as surgical thread or broken bone setting materials, hygienic articles such as diapers or sanitary articles, agricultural or horticultural materials such as mulch films, slow release chemicals, fishery materials such as fishing nets, packaging materials, and in many other fields.
Nevertheless, PHB has mechanical problems in that it has a high crystallinity (70% or more) and a poor flexibility and can be worked only with difficulty, since it is thermally decomposed at a temperature of the melting point (180.degree. C.) thereof or higher, and has a poor impact resistance. Further, because of a high production cost thereof, PHB has not been industrially produced on a practical scale.
Recently, studies have been made into the use of a copolymer basically comprising a 3-hydroxybutyrate (3HB) unit, and a process for the production thereof using Alcaligenes eutrophus.
For example, a copolymer comprising 3HB and 3-hydroxyvalerate units (3HV) is disclosed in (e.g., Japanese Unexamined Patent Publications (Kokai) Nos. 57-150393, 58-69224, 59-220192, 63-269989 and 64-69622.
The introduction of the 3HV component into the copolymer lowers the crystallinity and improves the flexibility of the same. Nevertheless, this copolymer still has problems in that the melting temperature is largely varied. For example, as the content of the 3HV component is increased from zero to 33 mol %, the melting temperature is rapidly lowered from 180.degree. C. to 85.degree. C. (see T. L. Bluhm et al., Macromolecules, 19, 2871-2876 (1986)), and therefore, it becomes difficult to industrially produce uniform products. This copolymer has other problems in that valeric acid, which is expensive, is used as a carbon source (Japanese Unexamined Patent Publication (Kokai) Nos. 63-269989 and 64-69622); and the production process is complicated (Japanese Unexamined Patent Publication (Kokai) No. 59-220192).
The terpolymer (or three unit copolymer) wherein, in addition to the above-mentioned 3HB and 3HV, a 5-hydroxyvalerate unit (5HV) is introduced as a third component is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 64-48820, and the terpolymer wherein a 3-hydroxypropionate unit (3HP) is introduced as a third component is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-69224.
The former terpolymer has a problem in that, although the melting point thereof is as low as about 100.degree. C., the effect of the melting point due to the introduction of the 5HV component is not observed because the melting point thereof is not substantially different from a copolymer composed of 3HB and 3HV containing the same mol % of 3HV. Further, also in the production thereof, 5-chloro-valeric acid used as a carbon source is expensive, and the polymer content in the microorganism cells is low.
The latter terpolymer has a high melting temperature of 170.degree. to 172.degree. C., and at a temperature higher than this temperature, it is easily thermally decomposed, and therefore, problems arise during a melt molding thereof.
Japanese Unexamined Patent Publication (Kokai) Nos. 64-48821 and 1-222788 disclose a copolymer comprising 3HB and a 4-hydroxybutyrate unit (4HB). These copolymers have a drawback such that the melting temperatures thereof are 156 to 159.degree. C., and thus a thermal decomposition during a thermal melt molding thereof occurs.
As described above, a practical thermoplastic copolymer having a flexibility, an excellent moldability, biodegradability and biocompatibility characteristics, and which is not thermally decomposed, is easily moldable and is produced by microorganisms, as well as a process for producing the same, are not conventionally known.
Japanese Unexamined Patent Publication (Kokai) No. 58-69224 teaches that the 4-hydroxyvalerate unit (4HV) used in the present invention can be introduced into a copolymer from 4-hydroxyvaleryl coenzyme A (i.e., CoA), which is derived from a condensation and reduction of acetyl CoA and acryl CoA or a condensation and dehydration of 3-hydroxy or 3-chloropropionate and acetyl CoA. Nevertheless, there are no specific descriptions of a copolymer containing a 4-hydroxyvalerate unit (4HV) in this publication, although only a possibility thereof is vagully suggested.